The present invention generally relates to semiconductor devices having a silicon-on-insulator (SOI) structure and production methods thereof, and more particularly to a semiconductor device having an SOI structure with a trench and a production method thereof.
In a semiconductor device which uses the SOI structure, it is possible to completely isolate elements using a trench isolation structure. For this reason, it is possible to improve the resistance against radioactive rays, prevent latch up, and increase the operation speed by the reduced substrate capacitance.
However, when a trench is formed in the SOI substrate, a crystal characteristic and an electrical characteristic of the semiconductor device deteriorate. Particularly in the case where the SOI substrate is formed by bonding two substrates, the characteristic deterioration is more likely to occur.
A description will be given of a conventional method of forming a trench in an SOI substrate which is formed by bonding, by referring to FIG. 1. FIG. 1 shows a cross section of an essential part of a semiconductor device which is made up of the SOI substrate.
The semiconductor device shown in FIG. 1 includes an active substrate 1 which is made of silicon (Si) and in which elements of the semiconductor device are formed, a first insulator layer 2 which is formed on the side of the active substrate 1 is made of silicon dioxide (SiO.sub.2), a base substrate 3 which is made of Si, a second insulator layer 4 which is formed on the side of the base substrate 3 is made of SiO.sub.2, and a trench 5. The first and second insulator layers 2 and 4 are bonded at a bonding surface 6.
When producing the semiconductor device shown in FIG. 1, the following steps are taken.
(1) The first insulator layer 2 is formed on a surface of the active substrate 1, while the second insulator layer 4 is formed on a surface of the base substrate 3.
(2) The active substrate 1 and the base substrate 3 are bonded by bonding the first and second insulator layers 2 and 4 together.
(3) The other surface of the active substrate 1 is polished to an appropriate thickness. For example, the appropriate thickness is in the range of 1 to 10 .mu.m.
(4) A resist process of a normal photolithography technique and a dry etching using Cl.sub.2 as the etching gas for etching Si or CF.sub.4 +CHF.sub.3 as the etching gas for etching SiO.sub.2 are used to form the trench 5 by selectively etching the active substrate 1 and the first and second insulator layers 2 and 4. At this stage, the trench 5 generally maintains the pattern which is determined by a photoresist mask which is formed during the resist process.
(5) A fluorine chemical is used to remove the etching residue or damage caused by the etching carried out in the step (4). By this step (5), the first and second insulator layers 2 and 4 are subjected to side etching and the trench 5 becomes spread at the lower part as shown in FIG. 1.
The step (5) must be carried out to remove the damage caused by the etching in the step (4), because the damage will diffuse further into the active substrate 1. When such damage diffuses further into the active substrate 1, the characteristic of elements formed in a vicinity of the trench 5 will deteriorate. On the other hand, when elements are not formed in the vicinity of the trench 5, the integration density of the semiconductor device becomes poor.
Therefore, according to the conventional method, the step (5) which uses the fluorine chemical must be carried out after forming the trench 5 by the step (4). But because of the step (5), the first and second insulator layers 2 and 4 are subjected to the side etching and an overhang part 1A is formed in the active substrate 1.
When the overhang part 1A exists in the active substrate 1, stress and electric field concentrate at a corner 1B of the overhang part 1A to cause crystal defects and insulation breakdown.
In addition, in the case of the bonded SOI substrate in particular, the active substrate 1 and the base substrate 3 may separate from each other at the bonding surface 6 while the fluorine chemical is used to remove the etching residue or damage caused by the etching.